Turbomachines operate by exchanging energy with a working fluid using alternating rows of rotating blades (hereinafter “turbomachinery blades”) and non-rotating vanes. Each turbomachinery blade interacts with the working fluid. Turbomachinery blades may be attached to and secured in a circumferential blade array to a rotor disk or other similar component of a gas turbine engine, or fan or compressor of a turbomachine.
It is known that turbomachinery blades (e.g., fan blades, propeller blades, compressor blades, turbine blades, etc.) are subject to destructive vibrations due to unsteady interaction of the blades with the working fluid. One type of vibration is known as flutter, which is aero-elastic instability resulting from the interaction of the flow of the working fluid over the blades and the blades' natural vibration tendencies. When flutter occurs, the unsteady aerodynamic forces on the blade, due to its vibration, add energy to the vibration, causing the vibration amplitude to increase. The vibration amplitude can become large enough to cause material fatigue failure of the turbomachinery blades unless the flutter is properly damped. The turbomachinery blades are stable (no flutter) when damping is positive. In addition to possible material fatigue failure, problems related to flutter may impose large costs and program delays as they are typically encountered late in development when engines or other turbomachinery are tested at full power or in flight conditions. The operating range of turbomachinery, in terms of pressure rise and flow rate, is restricted by various flutter phenomena. For example, turbomachinery blades that in use substantially operate in the transonic range (referred to hereinafter as “transonic turbomachinery blades”), such as transonic fan blades of transonic fans and compressors, are susceptible to transonic stall flutter, a flutter phenomenon that occurs with partial or complete separation of the flow of working fluid (in this case, airflow) about the transonic turbomachinery blade. Transonic fans and compressors are widely used in gas turbine engines because of their benefits in terms of compactness and reduced weight and cost. The transonic range may be defined as the range of working fluid (usually air) speed in which both subsonic and supersonic flow conditions exist around the transonic turbomachinery blade, and generally refers to an inlet Mach number, or relative inlet Mach number, between about 0.7 and about 1.0. As the transonic flow moves over the transonic turbomachinery blade, the flow is accelerated, becoming locally supersonic. Flowfields comprising supersonic flows, such as transonic flowfields, tend to produce the aero-elastic instability that is evidenced by flutter, including transonic stall flutter. Conventional transonic turbomachinery blades have a zero or near zero camber near the leading edge at cross sections where supersonic flow is expected and camber across the blade at cross sections where the flow is subsonic, near the hub where the rotational velocity is low. Conventional turbomachinery blades that are entirely subsonic will usually have camber across all cross sections from 0% to 100% span.
Hence, there is a need for flutter-resistant transonic turbomachinery blades and methods for reducing transonic turbomachinery blade flutter.